mirror of
https://github.com/rbock/sqlpp11.git
synced 2024-12-27 00:16:59 +08:00
56 lines
1.8 KiB
Markdown
56 lines
1.8 KiB
Markdown
# Introduction
|
|
Database NULL is a strange beast. It can be compared to anything but that comparison never returns true. It also never returns false, it returns NULL. Even
|
|
|
|
```SQL
|
|
NULL != NULL -> NULL
|
|
NULL = NULL -> NULL
|
|
```
|
|
|
|
A value like that would be pretty unusual in C++. Especially since fields in a result could be either a decent value or NULL. And this can change from result row to result row.
|
|
|
|
Also, in `where` or `having` conditions, you have to change the expression (not just parameters) if you want to NULL instead of a value:
|
|
|
|
```SQL
|
|
a = 7
|
|
a IS NULL
|
|
```
|
|
|
|
The library tries to make the usage of NULL reasonably safe in C++:
|
|
|
|
# Assigning NULL in INSERT or UPDATE
|
|
In assignments in `insert` or `update` statements, NULL can be represented by `sqlpp::null`.
|
|
|
|
```C++
|
|
db(insert_into(t).set(t.gamma = sqlpp::null);
|
|
```
|
|
|
|
Note that you cannot assign `sqlpp::null` to columns that are not nullable.
|
|
|
|
# Comparing to NULL
|
|
Columns that can be NULL can be compared to NULL using the `is_null` and `is_not_null` member functions.
|
|
|
|
```C++
|
|
const auto s = select(all_of(tab)).from(tab).where(tab.alpha == 17 and tab.beta.is_null());
|
|
```
|
|
|
|
# Obtaining potential NULL values from a select
|
|
sqlpp11 can determine whether a result field can be null, based on the columns involved and the structure of your query. If in doubt (for instance due to dynamic parts), it will assume that a field can be NULL.
|
|
|
|
You can check if a field is NULL by calling the `is_null()` method.
|
|
|
|
The value can be accessed by the `.value()` method. It will return a default constructed value in case `is_null() == true`.
|
|
|
|
```C++
|
|
for (const auto& row : db(select(all_of(tab)).from(tab).unconditionally()))
|
|
{
|
|
if (not row.alpha.is_null())
|
|
{
|
|
const auto a = row.alpha.value();
|
|
}
|
|
}
|
|
```
|
|
|
|
# Future:
|
|
We might switch to `std::optional` to represent nullable columns or values.
|
|
|